Thread Subject: how to assign lat/lon to a 2D Earth image

For the below earth image

http://veimages.gsfc.nasa.gov/2430/land_ocean_ice_2048.jpg

how to assign the latitudes/longitudes to the corresponding
locations of the image? For example, if I plot a red point
on the location (lon,lat) then the corresponding location on
the image will show a red point.

"Roy Chang" <tigermacross@yahoo.com> wrote in message <flheik$l3b
$1@fred.mathworks.com>...
> For the below earth image
>
> http://veimages.gsfc.nasa.gov/2430/land_ocean_ice_2048.jpg
>
> how to assign the latitudes/longitudes to the corresponding
> locations of the image? For example, if I plot a red point
> on the location (lon,lat) then the corresponding location on
> the image will show a red point.
--------
  That image looks very much like a Mercator projection. If so, then longitude
and latitude values can be translated linearly into the usual x,y indices for
images. In other words, the area within a one-degree interval of longitude and
one-degree interval of latitude will the same-sized rectangle - probably a
square - throughout the entire map.

Roger Stafford

Hi,
Could you please show me which commands I can use to load
the image file, assign the lat/lon, draw the figure, and
plot the result as I mentioned?
Thanks a lot.

"Roy Chang" <tigermacross@yahoo.com> wrote in message
<fljqls$ian$1@fred.mathworks.com>...
> Hi,
> Could you please show me which commands I can use to load
> the image file, assign the lat/lon, draw the figure, and
> plot the result as I mentioned?
> Thanks a lot.
>

Normally the operation you want (image registration or
georeferencing) is more complicated but in this case it's
trivial since the image is simply in lon-lat covering the
whole earth. So something like this should work

I = imread('the image'); % 'the image' is obviously
the image file name
image([-180 180],[-90 90],I)

J. Luis

"Roy Chang" <tigermacross@yahoo.com> wrote in message <fljqls$ian
$1@fred.mathworks.com>...
> Hi,
> Could you please show me which commands I can use to load
> the image file, assign the lat/lon, draw the figure, and
> plot the result as I mentioned?
> Thanks a lot.
---------
  Roy, I cannot advise you as to loading images, drawing the figure, or
plotting, since I don't have the imaging toolbox on my matlab version. I can
only indicate the conversion from latitude and longitude values to the
corresponding pixel coordinates of your image. You will note that in doing
so I am assuming that the projection in this image is actually a Mercator
projection as I guessed earlier in this thread, and that the image covers the
entire earth from pole to pole and east to west, as it seems to do.

  Let it be understood that 'lat' and 'long' are values that express earth's
latitude and longitude in degrees and fractions of a degree (not in minutes
and seconds,) with the understanding that latitude south and longitude west
values are to be negative. Thus 'lat' ranges from -90 to +90 and 'long'
ranges from -180 to +180. Since your image is 1024 by 2048 pixels, and
since the Greenwich prime meridian is apparently located at the east/west
center of the image, the conversion to pixel coordinates can be expressed by:

 f = 1-2*eps; % Keep indices within bounds
 row = ceil(512-lat*1024/180*f);
 col = ceil(1024+long*2048/360*f);

  For example, Mt. Whitney in the Sierra Nevada mountains is located at
latitude 36 deg, 34', 42.78" N and longitude 118 deg, 17', 32.604" W which
would make lat = 36.57855 and long = -118.29239. By the above
conversion this would lie within the pixel with coordinates (304,352).

Roger Stafford